Airborne wind energy (AWE) generally refers to producing energy from wind using a tethered air vehicle (i.e. a kite). A system of this type has a significant advantage over ground based wind turbines because it can take advantage of higher altitude winds which are faster and more reliable than surface level winds. Two known methods of implementing an AWE system are referred to as flygen and groundgen systems. Flygen involves putting a turbine and generator on a kite and transmitting power over wires to the ground. In contrast, groundgen systems use a kite to produce mechanical energy which turns a generator on the ground.
Flygen systems require a turbine and generator on board the kite, and a large kite capable of generating high lift is therefore required. Since the kite (which may resemble a glider aircraft) and its payload are relatively expensive, it is necessary to develop a sufficiently reliable control system to prevent crash landings. Control systems may utilize sensors such as GPS receivers and inertial measurement systems connected to an on-board autopilot. These components further increase the cost of the system.
On type of groundgen system utilizes the tension in the lines to turn a generator. If the kite is reeled out while flying (e.g. in a crosswind trajectory), the kite produces power equal to the product of line tension and line velocity. After the line is fully reeled out, the kite can be flown directly over its pilot on the ground, decreasing the line tension and allowing the kite to be reeled in at a very low line tension. Thus, the kite can be reeled in utilizing less energy than the kite generated as the line reeled out by controlling the trajectory of the kite as it is reeled in and out.
Airborne wind energy systems generally require high speed crosswind flight to generate high line tensions and thereby maximize power output. When a kite is flown perpendicular to the wind direction (e.g. in a circle or FIG. 8 pattern), its crosswind component of velocity is a factor of L/D higher than the airspeed in the wind direction (equation 1, below). For groundgen systems, this expression, along with an assumed line speed, can be used to find the airspeed of the kite, the lift, line tension, and finally power (equations 1-6). Assuming a high L/D ratio, neglecting parasitic drag, and using a small angle approximations results in va=vc, T=L, and approximate power production can be calculated (equation 7). A line speed of vl=vw/3 maximizes power (equation 8).
                              v          c                =                              (                                          v                i                            -                              v                ∞                                      )                    ⁢                      (                          L              /                              D                h                                      )                                              (        1        )                                          v          a                =                              (                                          v                l                            -                              v                w                                      )                    ⁢                                    1              +                                                (                                      L                    /                                          D                      k                                                        )                                2                                                                        (        2        )                                L        =                              1            2                    ⁢                      pC            L                    ⁢                      Au            a            2                                              (        3        )                                          T          i                =                                                            L                2                            +                              D                h                3                                              =                      L            ⁢                                          1                +                                  1                                                            (                                              L                        /                                                  D                          k                                                                    )                                        2                                                                                                          (        4        )                                          P          w                =                              1            2                    ⁢                      pv            w            3                                              (        5        )                                P        =                              v            i                    ⁢                      T            i                                              (        6        )                                P        =                              1            2                    ⁢                                                    pAC                L                            ⁡                              (                                  L                  /                                      D                    k                                                  )                                      2                    ⁢                                                    v                i                            ⁡                              (                                                      v                    l                                    -                                      v                    w                                                  )                                      2                                              (        7        )                                P        =                              2            27                    ⁢                                                    pAC                L                            ⁡                              (                                  L                  /                                      D                    k                                                  )                                      3                    ⁢                      v            w            3                                              (        8        )            
Where:
A=kite planform area
CL=coefficient of lift
Dk=induced drag
L=lift
P=power
Pw=wind power density
ρ=air density
Tl=line tension
va=airspeed
vc=crosswind component of velocity
vl=line speed
vw=wind speed
However, known kite control systems suffer from various drawbacks.